0206-reverse-linked-list/0206-reverse-linked-list.java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        return reverseNode(head, null);
    }
    
    public ListNode reverseNode(ListNode node, ListNode prevNode){
        if(node == null){
            return prevNode;
        }
        ListNode nextNode = node.next;
        node.next = prevNode;
        return reverseNode(nextNode, node);
    }
}